A Chinese billionaire was executed Monday after a court found him guilty of multiple murders and leading a mafia-style gang. He is only the latest high profile businessman with links to former security chief Zhou Yongkang to fall under China's crackdown on corruption.

Mining tycoon, Liu Han, his brother and three associates were put to death Monday morning. Liu Han had amassed a billion-dollar fortune as head of Hanlong Group, the largest private enterprise in Sichuan Province. He invested in minerals, electricity, energy, finance and real estate.

Liu, 48, had been found guilty of 13 crimes, including including murder, organizing casinos, running a mafia-style gang and selling firearms.

Joseph Cheng, a professor of International Relations at the University of Hong Kong, said Liu’s execution affirms Chinese President Xi Jinping’s willingness to go after “big tigers” in his crackdown on corruption in China.

“The execution of an extremely rich tycoon is very significant in the eyes of the Chinese people. Increasingly in recent years people in China believe that rich tycoons can buy a lot of influence,” Cheng said.

China’s Ministry of Public Security began its investigation into LiuI in March of last year.

Rise to power

Following his execution Monday, China’s state-run Xinhua news agency published a lengthy description of Liu’s rise to power, which according to Xinhua was helped by corruption, bribery and murder. Xinhua said nine people died at the hands of Liu’s henchmen, and five of those through gun shootings. But human rights activists, like William Nee at Amnesty International, have raised questions about China’s investigation into Liu and his brother Liu Wei.

“I think there are causes for concern. Liu Wei said in court that he was tortured while being interrogated, and he was beaten every day. So under Chinese law, and under criminal law, any evidence that is taken through torture should be excluded," Nee noted. "But it seems that in this case that probably wasn’t the case.”

Nee said the investigation raises broader questions about China’s corruption crackdown, in which government officials can be placed under shanggui, a form of extra-legal detention where investigation subjects are not allowed contact with friends or family, have no access to legal defense and are sometimes tortured.

Nee said that none of the targets of China’s crackdown on corruption will have protection under rule of law. “The fact that so many people are investigated outside of the formal criminal justice system is a key concern, especially because one of the key components of a fair trial is having proper legal representation,” he stated.

China’s Caixin magazine reported that Liu had had business dealings with Zhou Bin, the son of disgraced politician Zhou Yongkang.

Zhou, China's former security chief and Politburo member, has been accused of serious violations of party and organizational discipline and secrecy. He is awaiting prosecution.